Question: Why do people refer to the Hebrews/Israelites as Jews? As far as I know there were no reference to Jews in the old testament.
Answer: The term, Jew, is derived from Judah, son of Jacob. When the Kingdom of Israel was split after the rule of King Solomon, the southern area was known as the Kingdom of Judah and its inhabitants were called Judeans which eventually became Jews.
The Book of Esther refers to Mordecai as the Judean and the Jews of Persia at the time are referred to as Judeans. The term also occurs in a few other places in the Bible.
All the Best,
Rabbi Azriel Schreiber